The wilderness areas were maintained by not having a lot of people. US added 100M people in the last 40 years, and a lot of them seem to like the Western states' scenery, terrain, and weather.
1980 census shows just over 40M in the west:
https://www2.census.gov/prod2/statcomp/documents/1981-02.pdf
page 6
2019 figures show nearly 80M in the west:
https://www.census.gov/popclock/data_tables.php?component=gr...
https://www.pewtrusts.org/en/research-and-analysis/blogs/sta...